1. Technical Field of the Invention
The present invention relates generally to physical quantity sensors for sensing physical quantities such as force, pressure, torque, velocity, acceleration, impact strength, weight, mass, degree of vacuum, turning effort, vibration, and noise. More particularly, the invention relates to an improved structure of a physical quantity sensing element which ensures easy and reliable electrical connection of the sensing element with external devices and circuits.
2. Description of the Related Art
Physical quantity sensors, such as load sensors making use of piezoresistive materials and pressure sensors employing pressure-sensitive elements, have been widely used. The piezoresistive materials here denote materials which have the piezoresistive effect that when a stress, such as a compression stress, a tensile stress, a shearing stress, and a hydrostatic stress, is applied to the material, the electrical resistance of the material changes as a function of the stress.
Generally, a physical quantity sensing element, which composes a physical quantity sensor, detects a physical quantity as a pressure, generates an electrical signal indicative of the detected pressure, and outputs the generated electrical signal to external devices and circuits. Accordingly, the sensing element is essentially provided with means for outputting electrical signals, such as electrodes and terminals.
Such electrical signal outputting means for the sensing element may be provided in different manners depending on the structure of the sensing element.
For example, a conventional physical quantity sensing element has a structure where a pair of electrode layers is respectively disposed over each of the two major faces of a pressure-sensitive layer through which a physical quantity is to be applied to the pressure-sensitive layer, and a pair of insulating layers is respectively disposed over each of the two electrode layers. Further, the pair of electrode layers is so arranged as to protrude outwardly from the insulating layers, thus forming protruded portions of the electrode layers which serve as the electrical signal outputting means of the sensing element.
With such a structure, the electrode layers may become damaged when a large physical quantity is applied to the sensing element, so that the sensing element cannot provide a correct electrical signal representative of the applied physical quantity.
Moreover, Japanese Unexamined Patent Publication No. 2002-202209, the English equivalent of which is US Patent Application Publication No. 2004/0074306 A1, discloses another type of physical quantity sensing element.
The physical quantity sensing element includes a pressure sensing body that has an opposite pair of major faces perpendicular to the application direction of the physical quantity to be sensed, and an opposite pair of side faces parallel to the application direction of the physical quantity. The sensing element further includes a pair of pressure exerted bodies and a pair of electrodes serving as the electrical signal outputting means of the sensing element. The pressure exerted bodies are respectively disposed on each of the two major faces of the pressure sensing body, while the electrodes are respectively disposed on each of the two side faces of the same.
Since the electrodes are disposed on the side faces of the pressure sending body which are not subject to application of the physical quantity, they can be prevented from being damaged due to application of large physical quantities. However, with such an arrangement of electrodes, it becomes difficult to make electric connection of the sensing element with external devices and circuits by means of, for example, electrical terminals and wire bonding.